The invention relates to a handover method comprising the steps of measuring the level and/or quality of an uplink signal of a mobile station at a serving base station. The invention further relates to a mobile communication system in which a serving base station measures the level and/or quality of an uplink signal of a mobile station.
It is characteristic of cellular mobile communication systems that mobile stations may roam freely and switch from one cell to another within the area of a mobile communication system. If the mobile station MS has no call in progress, the cell crossover involves only registration with a new cell. If the mobile station MS has a call in progress during the cell crossover, the call must also be switched from one base station to another in such a way that the call is disturbed as little as possible. The latter procedure is called a handover. Handover may also take place within a cell from one traffic channel to another. To allow the mobile communication system to detect the need for handover and select a suitable target cell for handover, it is necessary to carry out various measurements of the quality of the radio connection and measurements of the field strengths of neighbouring cells. Handover from a serving cell to a neighbouring cell may take place, for example, when the measurement results of the mobile station/base station indicate a low signal level and/or quality for the currently serving cell, and a higher signal level can be achieved in a neighbouring cell. To ensure stability in a mobile communication network, the measurement results and parameters used in handover are averaged over a pre-set period of time. The averaging process makes handover less sensitive to measurement results distorted by instantaneous interference or fading.
FIG. 1 of the appended drawings shows a simplified block diagram of the pan-European GSM mobile communication system. A mobile station MS is connected over a radio path to a base transceiver station BTS, in the case of FIG. 1 to BTS3. A base station system BSS consists of base stations BTS controlled by a base station controller BSC. Each base station BTS comprises one or more transceiver units. A plurality of base station controllers BSC usually operate under the control of a mobile services switching center MSC. An MSC communicates with other MSCs and a public switched telephone network. The subscriber registers of the GSM system include at least a home location register HLR and a visitor location register VLR (not shown in FIG. 1). The subscriber data of a mobile subscriber are stored permanently in the HLR of the system and temporarily in the VLR in the area of which the MS is located at a given moment.
In order to enable a handover decision, a mobile station MS continuously measures downlink signals from the base stations BTS adjacent to the serving base station. In the GSM mobile communication network, for example, an MS may simultaneously measure the signal level and/or quality of both the serving base station and up to 32 other base stations. The serving base station informs the mobile station of the neighbouring base stations it should measure. The measurement results of each base station are identified on the basis of a base station identity code BSIC.
The MS sends the measurement results regularly in a report message through the serving base station BTS to the base station controller BSC. The report message contains the measurement results of the serving base station and up to six best neighbouring base stations. The base station controller BSC makes the handover decisions. However, if the target cell is controlled by another BSC, the handover is performed under the control of the MSC. Handover decisions may also be made in a centralized manner in the MSC. If necessary, the BSC sends a handover command to the MS through the serving BTS. According to the GSM recommendations, the rate at which measurement results are reported is at least once a second. In most applications, the MS reports the measurement results at intervals of 480 ms.
WO 92/09177 discloses a handover method for elongated cells located crosswise. According to this method, a radio telephone is handed over to a cell in the radio telephone""s direction of travel. The strength of signals from the surrounding cells is measured over a period of 30 to 60 seconds for a handover decision. The two strongest cells are determined on the basis of the measurement results, and the identities of these cells are stored in the radio telephone""s memory. If the cell that had the highest signal strength according to the measurements still has the highest or second highest signal strength after thirty seconds, a handover will be performed to this cell. Otherwise the currently serving cell is selected to continue the communication, and the measurement of signal strengths of the adjacent cells is also continued. The method is based on the idea that a cell along the direction of travel remains strong over the interval of time between measurements, whereas the signal strength of a cell that is perpendicular to the radio telephone""s direction of travel is high only for a short time. Since the method requires that a measurement is carried out for a certain period of time, it is not applicable to a fast handover, when the signal strength suddenly drops in the serving cell.
WO 95/35006 discloses a handover method based on the use of chained cells. In the mobile communication system disclosed, cells are arranged as chains of two or more cells, which allows a fast handover to a chained cell when the signal level suddenly drops in the serving cell, for instance when a high-speed mobile station suddenly moves behind an obstacle formed by the terrain or behind a building. The base station of the serving cell measures the uplink signal level of the mobile station. If the uplink signal suddenly drops below a predetermined threshold level, a fast handover is performed to one of the chained cells assigned to the serving cell. To ensure a sufficiently fast handover, the handover decision is made on the basis of instantaneous strength of the uplink signal. A mobile station constantly measures the downlink signal level of the neighbouring cells of the serving cell. If the serving cell has a plurality of chained cells, the handover is performed to the chained cell which, according to a few of the latest measurements, has the highest field strength. The problem is how to select the correct chained cell in an environment with several obstacles. Since the measurement results of the mobile station are averaged and there is a delay in the reporting, the measurement results are not up to date. The best chained cell selected on the basis of the latest measurements may remain behind another obstacle, in which case the signal level offered by it is no longer sufficient for maintaining a call.
The object of the present invention is to provide a controlled and reliable handover even in difficult environments with several obstacles.
This new type of handover is achieved with a method of the invention, which is characterized by measuring the level and/or quality of the uplink signal of the mobile station at neighbouring base stations, when the level and/or quality of the uplink signal has dropped below a predetermined triggering value at the serving base station, and performing handover to the neighbouring base station that has measured the most suitable uplink signal level and/or quality, when the uplink signal level and/or quality has dropped below a predetermined threshold value for handover at the serving base station.
The invention also relates to a mobile communication system of the type described in the introductory portion. The mobile communication system of the invention is characterized in that a base station controller is arranged to command neighbouring base stations to measure the level and/or quality of the uplink signal of the mobile station, when the serving base station has measured an uplink signal level and/or quality lower than a predetermined triggering value, and perform a handover to the neighbouring base station that has measured the most suitable uplink signal level and/or quality, when the serving base station has measured an uplink signal level and/or quality lower than a predetermined threshold value for handover.
The invention is based on the idea of measuring the level and/or quality of the uplink signal from a mobile station on a real time basis at the neighbouring cells in order to determine the best target cell for the handover.
In the mobile communication system of the invention, the uplink signal level and/or quality from the mobile station is measured at the neighbouring base stations before a handover, and the most suitable target cell is determined for the handover on the basis of these measurement results. Handover is anticipated when the level and/or quality of the uplink signal received at the serving base station drops close to the value used as the handover criterion.
One of the advantages of the fast handover method of the invention is that the best target cell for handover can be determined rapidly even in very difficult environments.
A further advantage of the handover of the invention is that it allows a call to be maintained more reliably by means of a handover to the best neighbouring cell, while the number of handovers is minimized.